1. Field of the Invention
The present invention relates to the broad field of sour gas/sulfur gas removal from coal combustion, coal gasification, or other product gas streams containing sulfur-laden pollutants. Process gas stream desulfurization, as it related to the present invention, generally means the removal of H.sub.2 S and/or COS from the process gas stream in a single step. The present invention teaches a process for the simultaneous removal of COS and H.sub.2 S from process gas streams by the reaction of such carbonyl sulfide with water vapor (to convert the COS to both CO.sub.2 and H.sub.2 S) and/or nitric oxide (to convert the H.sub.2 S to elemental sulfur) over a silica gel catalyst in a single reactor.
2. Description of the Prior Art
With the increased concern expressed, beginning in about the early 1970s, by various walks of life including the academia and the general public relating to the deleterious effects of air pollution came a paucity of research aimed at removing sulfur and nitrogen oxides from various emissions sources. Generally, treatment of these emissions can be placed in two categories: (a) those processes which selectively remove sulfur gases for purification and (b) those processes which convert these sulfur gases to elemental sulfur. Although many of these processes appear to have been adequately reviewed in the Kirk-Othmer Encyclopedia of Chemical Technology, Volume 22, pages 267-297, John Wiley and Sons, New York, 1983; brief discussion of some of the more salient points found in this literature is discussed below for the convenience of the reader.
The first category of processes, supra, normally utilizes an absorbent to remove the sulfur gases from the process gas stream. The resulting absorbent sulfur gas complex is subsequently treated, usually by simple heating, with oxygen or water to regenerate the absorbent and to recover a relatively purified sulfur gas. The absorbent in these processes can be either a solid metal or metal oxide either alone or on an inert support, or it can be containing an organic or inorganic absorbent with such solutions being aqueous- or nonaqueous-based. In reviewing the pertinent prior art, over 87 references were located which discussed minor improvements or changes in many of these processes. For the sake of brevity, only few of the more recent, representative, and pertinent of this plethora of prior art teachings are discussed below.
In 1987, the Electric Power Research Institute (EPRI) published findings of an investigation by Mittelhauser Corporation (G. E. Pack and G. L. Walter, Investigation of Sulfur Removal From Low-Pressure Gas, EPRI-AP-5102, April 1987) which compared six commercial processes for removing COS and H.sub.2 S from hot gases. These six processes were the Stretford, Shafer, Unisulf, Sulfolin, Lo-Cat, and Hiperion processes. All of these processes involve a liquid phase oxidation of the sulfur gases to elemental sulfur. Additionally, all of these processes require some form of gas purification or separation step prior to the oxidation step thereof. None of these six processes appeared superior for all sulfur gas removal operations, although the Lo-Cat process was selected as being better than the other five when utilized for gases containing low concentrations of sulfur gases.
The remaining literature references, supra, can be divided into the following groupings:
1. Processes using solid adsorbents. PA0 2. Processes using liquid absorbents. PA0 3. Processes in which the gases are reacted directly using a solid catalyst.
a. Processes using metal oxides/metals with or without supports. PA1 b. Processes using zeolites. PA1 a. Processes using organic absorbents. PA1 b. Processes using aqueous inorganic absorbents.
Processes using solid absorbents generally involve metals or metal oxides either alone, or on the surface of a silica or alumina support. For example, Denny and Wood (Eur. Pat. Appl. EP No. 243052, Oct. 28, 1987) use a bed of copper, zinc, or aluminum oxides for effectively removing sulfur compounds from natural gas under nonreducing atmospheres. E. Quemere uses zirconium or cerium with titanium supported on silica (EP No. 244310, Nov. 4, 1987) to remove sulfur gases from industrial waste gases. Nickel (BE No. 905986, Jun. 19, 1987), iron (Jpn. Kokai Tokkyo Koho JP No. 62/1436 A2, Jan. 7, 1987), and other metals are also used as supported or unsupported absorbents.
Sodium-calcium or sodium-A zeolites and molecular sieves are also used as absorbents (W. Lutz, M. Buelow, F. Zobel, and B. Kursmann, DD No. 242351, Jan. 28, 1987) for carbon dioxide contaminated with hydrogen sulfide.
Processes which use liquid absorbents generally use some type of organic amine which is regenerated in a second step, although other organics are also reported to be effective. For example, Dow Chemical Company has patented a process (Jpn. Kokai Tokkyo JP No. 62/106987, May 18, 1987) which uses alkanolamines such as methydietanolamine in an aqueous alkaline solution to remove COS, H.sub.2 S, CO.sub.2, and mercaptans from light hydrocarbons. Other processes use an aqueous inorganic base such as K.sub.2 CO.sub.3 [R. W. Rousseau, J. K. Ferrel, and J. S. Staton, Energy Prog. 7(1), 1-8] to remove COS and H.sub.2 S from coal gases.
In U.S. Pat. No. 4,314,977, Kulik, Feb. 9, 1982, there is described a process for the removal of either H.sub.2 S or NO from effluent gases. The procedule described therein requires a mole ratio H.sub.2 S:NO ranging from about 0.5:1 to about 1.1:1 and the addition of oxygen thereto such that the resulting oxygen concentration is greater than about one-half mole of oxygen per mole of NO. Under the above conditions either NO or H.sub.2 S can be removed completely from the gas stream. Although no clear-cut mechanism was suggested, the following two reactions are postulated as occurring: EQU H.sub.2 S+2NO.fwdarw.N.sub.2 O+S+H.sub.2 O (1) EQU 2H.sub.2 S+2NO+O.sub.2 .fwdarw.N.sub.2 +S+2H.sub.2 O (2)
References to processes in which the gases are directly reacted with a catalyst for conversion to alternate products also exist in the literature. For an example, a solid catalyst system has been patented by Klaus Storp and Emil Ruhi (Ger. Offen. DE No. 3535815 Al, Apr. 9, 1987) in which H.sub.2 S and SO.sub.2 are reacted with oxygen over a catalyst consisting of nickel impregnated alumina or titanium dioxide.
There has been some early work in which silica gel was used as a catalyst. In these early experiments, much lower conversions of the gases to sulfur were experienced than in the instant invention. This early work is discussed in greater detail below.
Early work by Pierce (Pierce, J. A., J. Phys. Chem. 1929, 33, 22-36) described the oxidation of hydrogen sulfide to elemental sulfur by nitric oxide in a glass reactor at low temperatures (28.degree.-100.degree. C.) and about 1 atm pressure over silica gel, glass wool, and ferric oxide-impregnated silica gel catalysts. Pierce suggested the mechanism given in equations 3 and 4, infra, in which two molecules of nitric oxide associate to form catalyst-adsorbed N.sub.2 O.sub.2, which N.sub.2 O.sub.2 serves as the actual oxidant for H.sub.2 S: EQU 2NO.fwdarw.N.sub.2 O.sub.2 ( 3) EQU N.sub.2 O.sub.2 +2H.sub.2 S.fwdarw.N.sub.2 +2H.sub.2 O+2S (4)
Pierce attributed a negative temperature effect to a decrease in the association of NO to N.sub.2 O.sub.2 at higher temperatures. In addition, he reported that blinding of the catalyst occurred by colloidal sulfur, resulting in an inhibitory effect.